Guanine nucleotide-binding proteins (GTP-binding proteins, or G proteins) participate in a wide range of regulatory functions including metabolism, growth, differentiation, signal transduction, cytoskeletal organization, and intracellular vesicle transport and secretion. These proteins control a diverse sets of regulatory pathways in response to hormones, growth factors, neuromodulators, or other signaling molecules. When these molecules bind to transmembrane receptors, signals are propagated to effector molecules by intracellular signal transducing proteins. Many of these signal transducing proteins are members of the Ras superfamily.
The Ras superfamily is a class of low molecular weight (LMW) GTP-binding proteins which consist of 21-30 kDa polypeptides. These proteins regulate cell growth, cell cycle control, protein secretion, and intracellular vesicle interaction. In particular, the LMW GTP-binding proteins activate cellular proteins by transducing mitogenic signals involved in various cell functions in response to extracellular signals from receptors (Tavitian, A. (1995) C. R. Seances Soc. Biol. Fil. 189:7-12). During this process, the hydrolysis of GTP acts as an energy source as well as an on-off switch for the GTPase activity of the LMW GTP-binding proteins.
The Ras superfamily is comprised of five subfamilies: Ras, Rho, Ran, Rab, and ADP-ribosylation factor (ARF). Specifically, Ras genes are essential in the control of cell proliferation. Mutations in Ras genes have been associated with cancer. Rho proteins control signal transduction in the process of linking receptors of growth factors to actin polymerization which is necessary for cell division. Rab proteins control the translocation of vesicles to and from membranes for protein localization, protein processing, and secretion. Ran proteins are localized to the cell nucleus and play a key role in nuclear protein import, control of DNA synthesis, and cell-cycle progression. ARF and ARF-like proteins participate in a wide variety of cellular functions including vesicle trafficking, exocrine secretion, regulation of phospholipase activity, and endocytosis.
Despite their sequence variations, all five subfamilies of the Ras superfamily share conserved structural features. Four conserved sequence regions (motifs I-IV) have been studied in the LMW GTP-binding proteins. Motif I is the most variable but has the conserved sequence, GXXXXGK. The lysine residue is essential in interacting with the .beta.- and .gamma.-phosphates of GTP. Motif II, III, and IV contain highly conserved sequences of DTAGQ, NKXD, and EXSAX, respectively. Specifically, Motif II regulates the binding of .gamma.-phosphate of GTP; Motif III regulates the binding of GTP; and Motif IV regulates the guanine base of GTP. Most of the membrane-bound LMW GTP-binding proteins generally require a carboxy terminal isoprenyl group for membrane association and biological activity. The isoprenyl group is added posttranslationally through recognition of a terminal cysteine residue alone or a CAAX motif. Additional membrane-binding energy is often provided by either internal palmitoylation or a carboxy terminal cluster of basic amino acids. The LMW GTP-binding proteins also have a variable effector region, located between motifs I and II, which is characterized as the interaction site for guanine nucleotide exchange factors (GEFs) or GTPase-activating proteins (GAPs). GEFs induce the release of GDP from the active form of the G protein, whereas GAPs interact with the inactive form by stimulating the GTPase activity of the G protein.
The ARF subfamily has at least 15 distinct members encompassing both ARF and ARF-like proteins. ARF proteins identified to date exhibit high structural similarity and ADP-ribosylation enhancing activity. In contrast, several ARF-like proteins lack ADP-ribosylation enhancing activity and bind GTP differently. An example of ARF-like proteins is a rat protein, ARL184. ARL184 has been shown to have a molecular weight of 22 kDa and four functional GTP-binding sites (Icard-Liepkalns, C. et al. (1997) Eur. J. Biochem. 246: 388-393). ARL184 is active in both the cytosol and the Golgi apparatus and is closely associated with acetylcholine release, suggesting that ARL184 is a potential regulatory protein associated with Ca.sup.2+ -dependent release of acetylcholine.
A number of Rho GTP-binding proteins have been identified in plasma membrane and cytoplasm. These include RhoA, B and C, and D, rhoG, rac 1 and 2, G25K-A and B, and TC10 (Hall, A. et al. (1993) Philos. Trans. R. Soc. Lond. (Biol.) 340:267-271). All Rho proteins have a CAAX motif which binds a prenyl group and either a palmitoylation site or a basic amino acid-rich region, suggesting their role in membrane-associated functions. In particular, RhoD is a protein which functions in early endosome motility and distribution by inducing rearrangement of actin cytoskeleton and cell surface (Murphy, C. et al. (1996) Nature 384:427-432). During cell adhesion, the Rho proteins are essential for triggering focal complex assembly and integrin-dependent signal transduction (Hotchin, N. A. and Hall, A. (1995) J. Cell Biol. 131:1857-1865).
The discovery of two new human Ras proteins and the polynucleotides which encode them satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention, and treatment of inflammation and disorders associated with cell proliferation and apoptosis.